Subsurface nickel boosts the low-temperature performance of a boron oxide overlayer in propane oxidative dehydrogenation

Oxidative dehydrogenation of propane is a promising technology for the preparation of propene. Boron-based nonmetal catalysts exhibit remarkable selectivity toward propene and limit the generation of COx byproducts due to unique radical-mediated C–H activation. However, due to the high barrier of O-H bond cleavage in the presence of O2, the radical initialization of the B-based materials requires a high temperature to proceed, which decreases the thermodynamic advantages of the oxidative dehydrogenation reaction. Here, we report that the boron oxide overlayer formed in situ over metallic Ni nanoparticles exhibits extraordinarily low-temperature activity and selectivity for the ODHP reaction. With the assistance of subsurface Ni, the surface specific activity of the BOx overlayer reaches 93 times higher than that of bare boron nitride. A mechanistic study reveals that the strong affinity of the subsurface Ni to the oxygen atoms reduces the barrier of radical initiation and thereby balances the rates of the BO-H cleavage and the regeneration of boron hydroxyl groups, accounting for the excellent low-temperature performance of Ni@BOx/BN catalysts.

Gao et al. report an active nickel-boron oxide catalyst for propane oxidative dehydrogenation. This particular catalyst can promote the reaction at temperatures relatively lower than regularly expected for conventional boron/boron nitride based catalysts with reasonably high selectivity propene. The authors show encapsulation of NI with a BOx overlayer yields the best performance, which is also modelled in the DFT calculations. However certain questions remain for the DFT portion of the work which are not convincing to explain the role of Ni in promoting the reaction. Overall these could be interesting results and are reasonably well-presented which are worth publishing in Nature Communications pending revisions Specific comments below: -Experimental thickness of boron oxide shell is 0.5-2 nm, corresponding to at least 2 atomic layers. How robust are the DFT conclusions for a two atomic layer model? For one layer, there is direct bonding between B and O with Ni, but this cannot be expected to be the case in the observed catalysts.
-In Fig 4, it would be helpful to mark transition states in the energy profile.
-I do not understand step G to H in the proposed mechanism: C-H abstraction is a barrierless process here, or have they simply not been calculated? Also, where is the second propane (C3H8*) on the surface? There is only one C3H7 shown in the inset image for state G.
-Based on this mechanism, the rate limiting step is G to H, with a thermodynamic barrier of 1.45 and probably an even larger kinetic one. This contradicts with the conclusion that the reason for higher activity is due to the regeneration of boron hydroxyls.
-How stable is the propyl spectator in the surface? Is this verified from in situ spectroscopy? What is the barrier of the second C-H activation to form propene on the surface? This mechanism would not hold if it is easier for propyl to just continue to react and leave as propene, and the surface B-O* radical would consequently no longer be present.
Reviewer #3 (Remarks to the Author): This contribution investigates the catalytic performance of boron oxide formed over metallic Ni nanoparticles in the oxidative dehydrogenation of propane (ODHP) by combined using of experimental and theoretical approaches. The authors successfully synthesize Ni@BOx/BN by using Ni-boron nitride hybrid materials after treatment at 800 °C in a CO2 flow and this catalyst shows a remarkable lowtemperature activity for ODHP compared to the conventional boron-based catalyst. In my view, this study provides another route for preparing a boron-based catalyst with improved activity and the catalytic properties have been well characterized. I have however some serious concerns as outlined below: 1. Although the Ni species encapsulated by BOx play a key role in the Ni@BOx/BN catalyst for ODHP, unlike the case of the Ni species, there is a lack of elucidation for understanding and supporting the role of B (e.g., BOx) species in the catalyst. Indeed, the catalysts with and without BOx species (i.e., Ni/BN-W and Ni@BOx/BN) exhibit the very different catalytic behaviors for ODHP. Thus, the details of the physicochemical properties of BOx in Ni@BOx/BN with the experimental data should be described more precisely. 2. As mentioned in this manuscript, the Ni@BOx/BN catalyst treated at 800 °C in a CO2 flow shows increased surface oxygen species. If the surface oxygen species formed by CO2 is attributed to BOx, which influences the catalyst activity, it is recommended to compare the catalyst properties treated under CO2 and O2, respectively. This will help to better understand the effect of BOx species on this reaction, and whether it can be controlled during the catalyst preparation step. Figures 2 and 3, the Ni@BOx/BN catalyst shows the better performance than the conventional boron-based catalyst. However, the catalyst performance in ODHP can be over-or underestimated due to the heat and mass transfer limitations (Org. Process Res. Dev. 2018, 22, 1644−1652). Therefore, more in-depth discussions of the catalytic results are required to clearly explain the transfer limitation in the Ni@BOx/BN catalyst system in comparison with the conventional one. 4. Figure 4 summarizes the computational results.

As shown in
Step A => B => C is considered an initiation step, generating the active species in the catalytic cycle. This step is very questionable. Subsequently, the authors let adsorbed O2 abstract an H-atom from BO-H to form both BO* and HOO* radicals. This step is problematic. The difference in BO-H and OO-H bond dissociation energy is likely much larger than the 0.25 eV (5.8 kca/mol) predicted. The authors did not benchmark their computational level for such open-shell species. Assuming O2 would be able to abstract that H-atom, why would it not directly abstract H-atoms from propane as the C-H BDE in propane is weaker than XO-H bonds? This part of the work is pretty shacky and needs to be addressed properly. 1

Response to the referees:
We thank the referees for the insightful and constructive comments and suggestions in order to help us further improve the manuscript. We have addressed all the comments point-by-point and revised the manuscript accordingly. In this response letter, comments from the referees are summarized in black typeface with the original comments quoted in italic, and our responses are in blue typeface. All major changes have been highlighted in blue in the main text. A detailed list of changes in the manuscript is also provided.
A list of main changes we have made: 1. We performed thorough investigation on influence of the heat/mass transfer and confirmed that the excellent performance of Ni@BOx/BN is its intrinsic catalytic behavior.
2. The performance of NiO/BN was evaluated to exclude the contribution of nickel oxide on the activity of Ni@BOx/BN catalyst.
3. The DFT calculation and data presentation was modified to explain the mechanism clearly. 4. The necessary data obtained during the revision was added into the manuscript and supporting information.
A point-to-point response to the comments Referee #1 The reviewer 1 confirms the significance of our research. However, reviewer considers that the structure catalyst under working conditions and after reaction needs to be identified, especially whether exposed nickel oxide species are formed.  Meanwhile, the NiO is an excellent low temperature catalyst for propane oxidation.
However, its propylene selectivity is very low in the ODHP reaction. From the recommended article (Topics in Catalysis, 63(19), 1731-1742.), the selectivity of propene of NiO is less than 20% under the C3H8 conversion of 5-20% (see Figure R3).
While, under the same conversion, the selectivity of propene of the Ni@BOx/BN catalyst is above 75%. Reply: It has been demonstrated the oxygen functionalization and formation of boron hydroxyl are common among boron based catalyst under the ODHP reaction condition. [1][2][3] Meanwhile, due to the low melting point of boroic acid, the surface of the working catalyst is highly dynamic. Therefore, the balance of leaching and SMSIdriven encapsulation of the working Ni@BOx catalyst will be built (Scheme R1). This phenomenon explains why the thickness of the core-shell catalyst is not identical.
Meanwhile, it also confirms that the BOx shell is loose enough to allow the reactant molecules to contact the inner shell of boron oxide. Therefore, all the BOx species may participate the reaction, but only the first one-or two-layers BOx shells are promoted by the subsurface Ni. In this sense, the simplification of Ni@BOx/BN with single layer of BOx covering the Ni NPs is appropriate, considering one of the main targets of this manuscript is to understand the promotion effect of encapsulated Ni NPs.  4 Certainly, the simulation of the core-shell catalyst with multiple BOx layers is important to evaluate the influence thickness of subsurface metal promoters. We will try to perform theoretical studies in the future studies.

Comment 5. "How stable is the propyl spectator in the surface? Is this verified from in situ spectroscopy? What is the barrier of the second C-H activation to form propene on the surface? This mechanism would not hold if it is easier for propyl to just continue to react and leave as propene, and the surface B-O* radical would consequently no longer be present."
Reply: For ODHP reaction over boron-based catalysts, researchers have done a lot of work to explore its mechanism, but it is still controversial due to the complicated reaction network. We have observed some features corresponding to the B-C bond in the in-situ IR spectra at 1200 to 1090 cm -1 , 7 which is the evidence of the existence of propyl species on the surface of catalysts ( Figure R4). We are not able to find the transition state for the cleavage of the second C-H bond from the B-C3H7. Therefore, we terminated the B atom with the propyl group in order to focus on the conversion of the boron hydroxyl, which is a reasonable simplification under low conversion. The barrier of the recombination of the B-O-B bond (C to A) is also much higher than the reaction cycles. Therefore, we think the recombination is not an important route under working condition. If taken the product water into consideration (high conversion), we found that the B-C3H7 is not stable as it will react with the product water to form another hydroxyl ( Figure R5, thermodynamically highly favorable with free energy change of -1.09 eV). Some papers 8,9 also reported that the B-C3H7 can react with gas phase C3H7O· radical to generate ·C3H7 radicals. While the B-O-C3H7 will further decompose to generate propene and B-OH. 8 In other word, if we consider the reaction network containing reactant and product water and other gas phase radicals, the most important step will still be the cleavage and regeneration of the O-H bond of boron hydroxyl. Figure R5. The thermodynamic calculation of the reaction between B-C3H7 species with water 8 We appreciate the reviewer's insightful comment and will try to evaluate the reaction network in the future studies. However, it can be anticipated that the work will be extremely time-and resource-consuming as the multiple layer BOx/Ni(111) is a very large system and numerous elementary steps should be considered.

Referee #3
This contribution investigates the catalytic performance of boron oxide formed over Reply: Thanks for the suggestions of the reviewer. In this manuscript, we demonstrated that the BOx overlayer is the active site for the excellent ODHP performance of Ni@BOx/BN catalyst and explained the function of the overlayer. We agree that the characterization of the structure of active site is important to obtain in-depth understandings on the catalytic performances. As a result, we tried to study the Ni@BOx/BN catalysts using various spectroscopic, diffraction and microscopy imaging methods and the results are shown in the manuscript and supporting information. From the characterization results, we confirm that the overlayer is 9 composed with amorphous boron oxide and the thickness of the shell is around 0.5~2 nm. However, the surface area of BOx overlayer is only 2% of the exposing surface area of the Ni@BOx/BN catalyst based on the chemisorption results. Considering thickness of the BOx overlayer is 0.5-2 nm (much smaller than the BN support), it is reasonable to anticipate that the molar percentage of BOx active species are is less than 0.1% in the bulk phase. Therefore, it is difficult to study the properties of BOx overlayer in detail using available bulk and surface sensitive techniques, especially when the B and O elements may also exist in large amount in the BN support, due to the insufficient sensitivity of diffraction and spectroscopic methods. The advanced electron microscopy coupling with chemical sensitive electron spectroscopic techniques and in-situ sample environment may provide the required local sensitivity to study the nature of the BOx overlayer under working condition. We will try to seek collaboration to perform the insitu characterization in the future.  for both catalysts, indicating that the intraparticle gradients play a minor role in the activity. It also shows that the reaction rate of Ni@BOx/BN catalyst is higher than that of BN at any particle size range.
To understand the effect of external mass transfer on the reaction rate, the weight of BN and Ni@BOx/BN catalysts were changed from 25, 50 to 100 mg in the ODHP reaction. The total gas flow was adjusted to obtain different W/F values. Fig.R8b displays the conversion of propane as a function of W/F value of the catalysts. With the increasing space velocity, the catalytic reactivity decreases for both BN and Ni@BOx/BN catalysts. Meanwhile, we also performed the reaction in the reactor with different inner diameter (6 mm and 10 mm, see Fig. R8c). At the same space velocity, the reaction rate in the 10 mm (ID) tube is much higher than that in the 6 mm tube, which is due to the large contribution of gas free radical pathways. This result also confirms that accelerating the radical initialization is an effective way to promote the activity of ODHP catalysts.
The Ni@BOx/BN catalyst shows significant activity advantages over the BN counterparts in all these experiments. Therefore, the subsurface Ni promotion effect on the BOx overlayer proposed in this manuscript is highly efficient in the ODHP reaction.  Figure 4 summarizes the computational results.
Step Reply: Thank for your suggestions. The mechanism of the ODHP reaction is very complicated, which involves both surface reaction and gas phase radical processes.
Therefore, the aim of the DFT calculation in this manuscript is to understand the promotion effect of subsurface metallic Ni particle, rather than to clarify the whole reaction networks. As a result, we make appropriate simplification in the theoretical calculations and mainly focus on the chemical variation of the active site in the radical initialization and regeneration steps.
Step A => B => C is the activation process, in which the boron hydroxyl active species generate. Although energy barrier from A to C is relatively high, it is still able to proceed for a reaction performed above 400 °C. Compared with the steps for the catalytic cycles, the recombination of the B and O atoms back into BOx shell is more difficult due to the relatively higher barrier. Therefore, the pre-activation step is reasonable. The IR spectra demonstrate that large amount of B-OH species formed after treating the catalyst with reaction atmosphere, which serves as a solid experimental evidence for the existence of the activation process.